Imaging spectroscopy in the visible and near infrared (VNIR) 400 nm-1000 nm and short wave infrared (SWIR) 1000 nm-2350 nm spectral ranges allows identifying potential targets as well as studying water and soil properties over large areas. NASA satellites carrying airborne hyperspectral imaging systems are used for of identifying a wide variety of surface materials by providing simultaneously many narrow, contiguous spectral bands and higher resolution reflectance spectra for every pixel in a spatial image.
Dyson type spectrometers are described in a paper “OPTICS FOR THE CANADIAN HYPERSPECTRAL MISSION (HERO)”. Journal: Sixth International Conference on Space Optics, Proceedings of ESA/CNES ICSO 2006, held 27-30 Jun. 2006 at ESTEC, Noordwijk, The Netherlands. Edited by A. Wilson. ESA SP-621. European Space Agency, 2006. Published on CDROM, p. 8.1. This paper describes a pushbroom Dyson type spectrometer. The spectrometer consists of two spectral channels VNIR and SWIR. The covered band width (spectrum) is 450 nm-2500 nm. The spectrometer incorporates two different gratings spaced from each other by approximately 0.5 meter and two slits. Large volume is needed for spectrometer packaging. Dyson prism blocks make the system heavy. The FPA (focal plane array) is made for 30 μm pixel size. The spectrometer F# is 2.2.
Spectrometer thermal properties are extremely important for the remote sensing platform configuration and packaging. For temperature sensitive spectrometers, special motor adjusting focal plane array position according to the temperature change is required. That leads to the increase of the packaging envelope and weight of carrying on the airborne platform.
A VNIR/SWIR spectrometer is disclosed in Patent Application US 2014/0071449, Mar. 13, 2014 High Efficiency Multi-channel Spectrometer. The VNIR/SWIR Offner type spectrometer optical system requires substantial envelope and includes multiple mirror, beam splitter, and pair of diffraction gratings.
U.S. Pat. No. 8,520,204, Aug. 27, 2013, discloses a Dyson Type Imaging Spectrometer having Improved Image Quality and Low Distortion. The entry or exit port is located inside the prism (or hemispherical lens)—there is no access for maintenance. No spectral correction over the wide spectrum is performed.
Another spectrometer is disclosed in U.S. Pat. No. 7,609,381, Oct. 27, 2009 Compact High-Throughput Spectrometer apparatus for Hyperspectral Remote Sensing. Only LWIR or VNIR spectral bands may be used, the spectrometer does not feature a combined VNIR/SWIR channel. A single refractive material is used for the VNIR channel, and chromatic aberration is not corrected.
Yet another spectrometer is disclosed in U.S. Pat. No. 7,768,642, Wide Field Compact Imaging Catadioptric Spectrometer, Aug. 3, 2010. This patent describes a wide field LWIR spectrometer with a pupil lens for the wide field aberration correction in the LWIR spectral band.
Another spectrometer is disclosed in U.S. Pat. No. 6,980,295 issuing to Lerner on Dec. 27, 2005 and entitled Compact Catadioptric Imaging Spectrometer Utilizing Reflective Grating. Therein disclosed is an imaging spectrometer having an entrance slit, an aspheric lens, grating, and detector. The spectrometer is designed to accommodate LWIR band width 8-13.5 μm.
Yet another spectrometer is disclosed in U.S. Pat. No. 7,006,217 issuing to Lerner on Feb. 28, 2006 and entitled Compact Catadioptric Imaging Spectrometer Utilizing Immersed Gratings. Therein disclosed is a spectrometer having an entrance slit, a catadioptric lens, a grating that diffracts the light back to the catadioptric lens and a detector. The spectrometer is designed for LWIR band width.
Yet another spectrometer is disclosed in U.S. Pat. No. 8,339,600, Dec. 25, 2012; Dual Waveband Compact Catadioptric Imaging Spectrometer. The spectrometer incorporates ZnSe grating, a curved sapphire prism, and BaF2 catadioptric lens or dioptric lens with a mirror, the F# is 4.
Even though there has been substantial development in the field of spectrometers for hyperspectral imaging, there is a need for a compact athermal VNIR/SWIR hyperspectral imaging spectrometer. Specifically, there is a need to provide a compact and efficient VNIR/SWIR imaging spectrometer with low F# and wide field of view.